Abstract

The aeolian stratigraphic record on the coast of central Chile (31°-35°S) includes Pleistocene, Holocene, and actual generations with paleodune sequences separated by interstratified paleosols. Previous work has associated these units with paleoclimatic changes linked to the oscillation and intensification of Southern Westerly Winds (SWW). Paleosols have been related to humid conditions (SWW shifted to the north) and paleodunes with arid conditions (SWW shifted to the south). Therefore, these eolian deposits are direct proxies of the paleoclimate of central Chile, where we lack a complete understanding of the Pleistocene. The current dune fields have been mapped north of the mouth of important rivers that would transport sands to the sea, where the coastal drive would take them to the north, eventually deposited on the beaches and then deflated into dune fields. This spatial distribution of the dune phenomenon leads to relating the origin of the sands to fluvial processes as the source of sediments. However, these approaches have not been investigated in detail, and questions remain: When do coastal dunes in central Chile form? What rock formations provide the sediments that constitute the dunes? What role do Andean glaciations play in dune morphogenesis? Therefore, a central focus of our research is to understand and determine the source of the sands that form the paleodune fields and their paleoenvironmental implications. We present our methodological approach together with preliminary research results for the Aconcagua (32°S) and Rapel (34°S) river basins. The prospected units correspond to paleodunes, present-day dunes, and fluvial terraces in each basin to satisfy a geographical criterion that links fluvial, marine, and aeolian processes. Our study sites include the paleodunes of Ventanas II, Ritoque dunes, and the lower fluvial terrace of the Aconcagua River. Ventanas II exposes a detailed stratigraphic sequence of 15 m with 13 well-developed units overlying the Horcón Formation (Miocene-Pliocene). For the Rapel basin, the paleodunes of the Brisas de Santo Domingo, the Santo Domingo dunes, and the lower terraces of the Rapel River are studied. In these units, we have collected detailed stratigraphic information (to understand the sedimentary processes), dated sands by Optically Stimulated Luminescence (to determine the age of formation of paleodunes and to estimate the age of paleosols), and U-Pb dating in detrital zircons (to determine the source of sediments). Based on our results in stratigraphy and U-Pb dating, we observe not only an Andean affinity but also other main sources of sands in the hinterland that change their relative importance with time. Our results indicate that the sands come mainly from rivers located immediately south of the dune fields. We discuss our preliminary results regarding the source and supply of sands and the paleoenvironmental implication of the paleodune records of the coast of central Chile.